1. Field of the Invention
The invention relates to the manufacture of an insulated electric wire having a uniform outside diameter in a wire insulating line where an extruder extrudes an insulating material around a core comprising at least one conductor and a take-up device takes up a finished insulated wire. More particularly, the invention relates to methods of and apparatus for controlling an outside diameter of the insulated wire during a transitional period upon every startup of the wire insulating line.
2. Description of the Prior Art
In a wire insulating line, major factors that determine the outside diameter of an insulated wire comprising at least one core conductor of a uniform diameter are extruder speed and wire take-up speed. When the extruder speed is increased or the take-up speed is reduced, the amount of insulating material supplied per unit length of the core conductor increases, resulting in an increase in the outside diameter of the finished insulated wire. Contrary to this, when the extruder speed is reduced or the take-up speed is increased, the amount of insulating material supplied per unit length of the core conductor decreases, resulting in an reduction in the outside diameter of the finished insulated wire.
In one known arrangement, a diameter measuring device continuously monitors the outside diameter of the finished insulated wire and measurement results are fed back for controlling either the extruder speed or the take-up speed so that the insulating layer is formed to a desired outside diameter.
This arrangement, however, is effective only after both the extruder and take-up device have reached steady-state running conditions. During a transitional period, or a period after the extruder and take-up device have been started up until they are stabilized enough to provide stable operation, difficulty would be found in controlling the outside diameter of the finished insulated wire. This is because optimum balance between the extruder speed and take-up speed in the transitional period is affected by a variety of factors such as operational characteristics of the extruder and take-up device, properties of the insulating material, diameter and type of the core conductor and desired thickness of the insulating layer. Particularly because the extruder has a structure in which a rotating screw pushes out a gelatinized insulating material around the core conductor, it is not certain that the actual extrusion rate is in perfect proportion with the revolving speed of a motor that drives the extruder screw. In addition, there exists a lag between the time a certain motor speed is set and the time the set motor speed is achieved, especially during the transitional period.
For reasons stated above., the startup procedure of the conventional wire insulating line usually requires a highly skilled operator. Further, the wire insulating line typically requires a considerably long time after power-on until its operation is stabilized enough to satisfy a required level of diameter tolerance, eventually causing a drop in production yield.
An approach widely employed in conventional wire insulating lines to overcome the aforementioned problem is to set a fixed target value for the ratio between the extruder speed and take-up speed based on experience. An operator manually starts and accelerates either the extruder or take-up device until a final line speed is achieved. Although this approach has proved effective to a certain extent, it is workable in rather limited conditions. Since the speed ratio is fixed in this approach, it is extremely difficult to follow or respond to varying operating conditions in a flexible manner, often leading to unsuccessful results.
Another previous approach directed to the above problem is disclosed in Japanese Patent Publication No. 62-177810, in which the diameter of the finished insulated wire is predicted from the extruder and take-up speeds measured at specified sampling intervals and the extruder speed setting is corrected based on a comparison between the predicted and measured diameters of the insulated wire. This approach is superior to the above-described approach in that the extruder speed is controlled based on the measured diameter of the insulated wire during the transitional period. According to the disclosure, however, a desired extruder speed is obtained as a function of the take-up speed and the take-up device requires manual intervention to set the rate of speed increase during the transitional period. In addition, too high a setting of acceleration of the take-up device could make it difficult for the extruder to catch up with the line speed and, therefore, a line operator is obliged to set a .rather moderate speed increase to avoid inconvenience. Consequently, the line startup time could not be much shortened and improvements in production yield and manufacturing efficiency would be negligible. Further, operation for increasing the take-up speed at an optimum rate would be extremely complicated according to this approach since a wide range of individual factors including the type of insulating material and other insulating conditions must be taken into consideration.